Polymer-coated carbon nanotube

ABSTRACT

By coating the outer surface of carbon nanotubes with various polymers of different properties, such properties as insulation property, reactivity, optical visibility, solvent dispersion property and so on are given to the outer surface of the carbon nanotubes.

TECHNICAL FIELD

The present invention relates to a carbon nanotube whose outer surfaceis coated with a polymer. Since such a carbon nanotube can be madeoptically visible, modified with biologically active substances andimproved in its physical properties, application of such a nanotube toextremely fine biosensors, fine processing apparatuses for cells, genetransfer, three-dimensional electronic circuits and so on is expected.

BACKGROUND ART

Carbon nanotubes are tube-like molecules 1 nm to several ten nm indiameter and 0.1 to several μm in length, which are classified into themono-layer type and the multi-layer type depending on the number oflayers of the walls constituting their hollow structures. Thesemolecules are expected as promising mechanical/electronic materialsbecause they are extremely thin and long, high in mechanical strength,excellent in electrical properties, and high in structure stability. Atpresent, they are mainly used as materials for engineering purposes, andare now being put into practical use as highly efficient electronradiation sources and probes for atomic force microscopes. Althoughstill in the research stage, application of carbon nanotubes to such asmicroactuators or gas sensors is now being studied. On the other hand,in the field of biochemistry, their application to probes for genetransfer or micromanipulators is being studied. However, in any of theirusages, electron microscopes are generally used for mechanical handlingof carbon nanotubes because of difficulty in their optical recognitionsince their diameters are below the limit of optical resolution.Further, methods of modification of carbon nanotubes currently knownare, for example, a method in which the carboxyl group generated at thesite of cleavage of the structure is used; a method using hightemperature fluorine; and a method in which defects are generated bysonication in polymethyl methacrylate.

Carbon nanotubes have such properties that they are extremely thin,excellent in structure stability, high in mechanical strength, and goodin electrical properties. However, on the other hand, their propertiesgive rise to the following problems: it is difficult to opticallyrecognize them because their diameters are below the limit of opticalresolution; it is difficult to process them because they are high inmechanical strength; it is difficult to introduce into them groups to bereacted with biologically active substances because they are high instructure stability; and it is necessary to insulate them when they areused as signal cables. In particular, in the application of them toorganisms where operations under microscopic observation areindispensable, their poor optical recognition property is a bigdrawback.

It is an object of the present invention to solve the above-describedproblems of the relevant technology and to provide novel carbonnanotubes which have overcome the conventional drawbacks.

DISCLOSURE OF THE INVENTION

As a result of extensive and intensive researches toward the solution ofthe above-described problems, the present inventors have found that, bydissolving a polymer which is expected to have affinity for a carbonnanotube in a solvent and then adding the carbon nanotube to thesolvent, the polymer is adsorbed onto the carbon nanotube to therebyreduce the optical absorption in the solution. The inventors haveassumed that this adsorption is attributable to the coating of the outersurface of the carbon nanotube with the polymer. Further, the inventorshave elucidated the state of coating of the carbon nanotube outersurface with a polymer using optical microscopes and electronmicroscopes. As a result, the inventors confirmed that is it possible tocompletely coat a carbon nanotube with a polymer. Further, sincepolymers are excellent in insulation, fluorescence property, easyintroduction of various reaction groups, and crosslinking/polymerizingreactivity, possibilities of improving the physicochemical properties ofcarbon nanotubes and adding thereto optical visibility by utilizing theabove properties of polymers have been found. Thus, the presentinvention has been achieved.

The first aspect of the present invention relates to a carbon nanotubewhose outer surface is coated with a polymer.

The second aspect of the invention relates to a method of preparing apolymer-coated carbon nanotube, comprising dissolving in a solvent apolymer having affinity for the outer surface of a carbon nanotube andthen suspending the carbon nanotube in the solvent to thereby coat theouter surface of the carbon nanotube with the polymer.

The third aspect of the invention relates to a method of making a carbonnanotube optically visible, comprising coating the outer surface of thecarbon nanotube with a polymer which emits fluorescence of the visiblelight wavelength range.

The fourth aspect of the invention relates to a method of preparing amaterial for three-dimensional electron wiring, comprising using apolymer-coated carbon nanotube.

The fifth aspect of the invention relates to a method of preparing acarbon nanotube with a high solvent dispersion property, comprisingcoating the outer surface of the carbon nanotube with a polymer.

Hereinbelow, the present invention will be described in more detail.

The carbon nanotube of the invention is a carbon nanotube whose outersurface is coated with a polymer. Although carbon nanotubes are readilyaggregated in a solvent, their dispersion property is improved bycoating them with a polymer. Besides, by using the polymers describedlater, it is possible to add specific properties to a carbon nanotube.

The carbon nanotube to be coated may be either a single-wall carbonnanotube or a multi-wall carbon nanotube.

The polymer used in the invention may be any polymer as long as it hasaffinity for carbon nanotubes. However, it is preferable to use polymerswhich have Si atom(s) or π conjugated bond(s) in the molecularstructure. Among such polymers, it is preferable to use thepolydibenzodisilaazepine represented by general formula (I) below.

(wherein R₁ to R₄ independently represent an alkyl group, aryl group,alkoxy group or aryloxy group each of which may be substituted; R₅represents a hydrogen atom or an alkyl group, aryl group, alkoxy groupor aryloxy group each of which may be substituted; and n represents thedegree of polymerization.)

Examples of the alkyl group in R₁ to R₅ in the above general formula (I)include straight-chained, branched or circular alkyl groups with 1-20(preferably 1-10) carbon atoms, such as methyl, ethyl, n- or iso-propyl,n-, iso- or tert-butyl, n-, iso- or neo-pentyl, n-hexyl, cyclohexyl,n-heptyl and n-octyl. Examples of the alkoxy group includestraight-chained, branched or circular alkoxy groups with 1-20(preferably 1-10) carbon atoms, such as methoxy, ethoxy, n- oriso-propoxy, n-, iso- or tert-butoxy, n-, iso- or neo-pentoxy, n-hexoxy,cyclohexoxy, n-heptoxy and n-octoxy. Examples of the aryl group includearyl groups with 6-20 (preferably 6-14) carbon atoms, such as phenylgroup, o-, m- or p-tolyl group, 1- and 2-naphtyl group, and anthrylgroup. Examples of the aryloxy group include aryloxy groups with 6-20(preferably 6-14) carbon atoms, such as phenoxy group, o-, m- orp-tolyloxy group, 1- and 2-naphtoxy group, and anthryloxy group.

As the polymer, any of the following polymers (1) to (4) may also beused.

(1) Polymers which Emit Fluorescence of the Visible Light WavelengthRange

Examples of such polymers include, in addition to thepolydibenzodisilaazepine described above, polythiophene,poly(N-vinylcarbazole) and poly(phenylene vinylene). Although carbonnanotubes have had the problem of difficulty in optical recognition,this problem can be solved by coating them with a polymer which emitsfluorescence of the visible light wavelength range.

(2) Polymers which Have on their Surface an Exposed Reaction GroupHaving Affinity for Biologically Active Molecules

Examples of such polymers include, in addition to thepolydibenzodisilaazepine described above, polyaniline,poly(vinylpyridine) and poly(vinyl alcohol). Examples of exposedreaction groups having affinity for biologically active moleculesinclude, but are not limited to, vinyl group, amine group, carboxylgroup, aldehyde group and hydroxyl group. Although carbon nanotubes havehad the problem of difficulty in introducing biologically activemolecules because of their high structure stability, this problem can besolved by coating them with a polymer which has on its surface anexposed reaction group having affinity for biologically activemolecules.

(3) Insulating Polymers

Examples of such polymers include, in addition to thepolydibenzodisilaazepine described above, polyethylene, polystyrene andpolyethylene terephthalate. Although it has been necessary to insulate acarbon nanotube when it was used as a signal cable, this problem can besolved by coating the carbon nanotube with an insulating polymer.

(4) Polymers which Have a Nature of Binding Firmly Through Crosslinkingor Polymerization

Examples of such polymers include, in addition to thepolydibenzodisilaazepine described above, polyarylene ethylene,polydiacetylene and poly(vinylpyridine). By using a polymer which has anature of binding firmly through crosslinking or polymerization forcoating, it becomes possible to enhance the intensity of the coatingmaterial (polymer), which leads to the preparation of a carbon nanotubewhose coating is not easily fallen off.

The carbon nanotube of the invention can be prepared, for example, bydissolving in a solvent a polymer which has affinity for the outersurface of a carbon nanotube and then suspending the carbon nanotube inthe solvent to thereby coat the outer surface with the polymer.

The solvent used here may be any solvent as long as it is capable ofdissolving the polymer. For example, when polydibenzodisilaazepine isused as a polymer, 1,2-dichloroethane, tetrahydrofuran or the like maybe used.

It is preferred that the carbon nanotube suspended in the solvent becompulsively dispersed to improve the reaction with the polymer. It hasbeen confirmed in experiments that carbon nanotubes are easilyaggregated after dispersion. It has been also confirmed that thepresence of a polymer improves the dispersion property of a carbonnanotube.

The amount of polymer to be dissolved for coating is not particularlylimited. For example, when polydibenzodisilaazepine is dissolved in1,2-dichloroethane, 0.1-20 mg/l is appropriate. The appropriate amountof carbon nanotube is 1 mg or less for 20 ml of the polymer solution.

The polymer-coated carbon nanotube may be taken out by dropping themixed solution of the polymer and the carbon nanotube onto a siliconbaseboard or slide glass and then drying. At this time, if the affinityof the polymer for the carbon nanotube is high, no particular problemwill occur. If the affinity is low, it has been observed that thepolymer layer adsorbed onto the carbon nanotube is thin when there isonly one solvent that is dissolving the polymer. Besides, it has alsobeen confirmed that microscopic observation of the polymer-coated carbonnanotube becomes impossible because a large quantity of dissolvedpolymer present in the background reduces the contrast of the nanotube.In order to solve this problem, a solvent that does not dissolve thepolymer much, such as ethanol or dimethyl sulfoxide, may be used toincrease the adsorption of the polymer onto the outer surface of thecarbon nanotube. Alternatively, similar effect may be obtained bydecreasing the temperature to thereby increase the aggregation propertyof the polymer and promote the aggregation thereof on the outer surfaceof the carbon nanotube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the changes in absorbance after the additionof a carbon nanotube to a solvent dissolving a polymer.

FIG. 2 presents photographs showing the results of fixed pointobservation of a polymer-coated carbon nanotube by different observationmethods. A is a light field image; B is a scanning electron microscopicimage; and C is a dark field image.

FIG. 3 presents photographs taken with a scanning electron microscope,showing the state of a carbon nanotube before and after coating with apolymer. a. shows the state before coating and b. shows the state aftercoating.

FIG. 4 is a photograph showing a comb-shaped electrode on which apolymer-coated carbon nanotube is oriented.

BEST MODES FOR CARRYING OUT THE INVENTION

Poly(5,10,10,11,11-pentamethyldibenzodisilaazepine-2,8-diyl) (generalformula (I), where R₁=R₂=R₃=R₄=R₅=methyl) was dissolved in1,2-dichloroethane to prepare a polymer solution (0.2 mg/l).Subsequently, to 5 ml of this solution, 0.01 mg of a multi-wall carbonnanotube prepared by the arc method was added. The mixture was agitatedby sonication for 1 minute and then left stationary. Absorbance wasmeasured immediately thereafter and 1 hour thereafter, followed bycomparison of absorption spectra (FIG. 1). As shown in FIG. 1, in thewavelength range below 350 nm where absorption characteristic topolydibenzodisilaazepine exists, the characteristic absorption no longerexists after 1 hour. Therefore, it was reasoned that the polymer wasadsorbed onto the carbon nanotube specifically. The multi-wall carbonnanotube used here usually has a diameter of approximately 100 nm and isarranged in bundles. With this thickness, it is barely possible toperform light field microscopic observation. Thus, such observation wasused for the confirmation of the state of polymer coating.

Based on the results obtained so far, the same site of the same samplewas examined by dark field microscopic observation, light fieldmicroscopic observation, and scanning electron microscopic observationat 370-450 nm which is the wavelength of the fluorescence emitted by thepolymer. Thus, the state of adsorption of the polymer onto the carbonnanotube was examined. The suspension of the polymer-coated carbonnanotube was dropped onto a silicon baseboard provided withcheckerboard-like divisions on which numbers are carved by etching.Using the thus provided number, the same site of the board was examinedby the above described three observation methods. As a result, a similarstring-like or collective-state nanotube was recognized at the same sitein each of the observation methods. Therefore, it was confirmed that thepolymer was adsorbed onto the entire surface of the carbon nanotube(FIG. 2). Further, the results of high resolution observation with ascanning electron microscope revealed that the shape of the outersurface of the carbon nanotube was clearly different before and afterthe polymer coating; it was confirmed that a large amount of polymer wasadsorbed onto the surface (FIG. 3).

Subsequently, a single-wall carbon nanotube was coated with the polymerin the same manner as described above, followed by measurement ofelectric properties. It is known that carbon nanotubes show the natureof conductor or semi-conductor. Thus, it is possible to orient a carbonnanotube on electrodes in an AC electric field. Also, since the crossingof both positive and negative electrodes by a carbon nanotube causesconduction of electric current, the electric properties of the abovepolymer-coated carbon nanotube can be confirmed by examining the amountof the voltage decrease from the applied voltage. To a comb-shapedelectrode 10 μm in electrode width and 5 μm in gap between electrodes,AC voltage of 9.0 Vp-p, 1 MHz was applied through a fixed resistance of1 kΩ, and 100 μl of the suspension of polymer-coated carbon nanotube wasdropped onto the electrode to thereby orient the carbon nanotube (FIG.4). As a result of measurement of the voltage decrease, no decrease wasrecognized though a number of carbon nanotubes were crossing bothpositive and negative electrodes. Therefore, it was confirmed that thecarbon nanotube was insulated by coating with the polymer.

The present specification encompasses the contents disclosed in thespecification and/or drawings of the Japanese Patent Application No.2002-160931 based on which the present application claims priority. Allpublications, patents and patent applications cited herein areincorporated herein by reference in their entity.

INDUSTRIAL APPLICABILITY

As described so far, by coating the outer surface of a carbon nanotubewith a polymer, it becomes possible to give the outer surface of thecarbon nanotube insulation property, reactivity, optical visibility andsolvent dispersion property.

1. A carbon nanotube whose outer surface is coated with a polymer. 2.The carbon nanotube according to claim 1, wherein the polymer emitsfluorescence of the visible light wavelength range.
 3. The carbonnanotube according to claim 1, wherein the polymer has on its surface anexposed reaction group having affinity for a biologically activemolecule.
 4. The carbon nanotube according to claim 3, wherein theexposed reaction group having affinity for a biologically activemolecule is a group selected from vinyl group, amine group, carboxylgroup, aldehyde group or hydroxyl group.
 5. The carbon nanotubeaccording to claim 1, wherein the polymer is an insulating polymer. 6.The carbon nanotube according to claim 1, wherein the polymer has anature of binding firmly through crosslinking or polymerization.
 7. Thecarbon nanotube according to claim 1, wherein the polymer has Si atom(s)or π conjugated bond(s) in its molecular structure.
 8. The carbonnanotube according to claim 7, wherein the polymer having Si atom(s) orπ conjugated bond(s) in its molecular structure is thepolydibenzodisilaazepine represented by general formula (I):

(wherein R₁ to R₄ independently represent an alkyl group, aryl group,alkoxy group or aryloxy group each of which may be substituted; R₅represents a hydrogen atom or an alkyl group, aryl group, alkoxy groupor aryloxy group each of which may be substituted; and n represents thedegree of polymerization.)
 9. The carbon nanotube according to claim 8,wherein R₁ to R₅ in general formula (I) independently represent a methylgroup.
 10. A method of preparing a polymer-coated carbon nanotube,comprising dissolving in a solvent a polymer having affinity for theouter surface of a carbon nanotube and then suspending the carbonnanotube in the solvent to thereby coat the outer surface with thepolymer.
 11. A method of preparing a polymer-coated carbon nanotube,comprising dissolving in a solvent a polymer having affinity for theouter surface of a carbon nanotube, suspending the carbon nanotube inthe solvent, and then increasing the amount of adsorption of the polymeronto the outer surface by adding thereto another solvent with a lowpolymer dissolution or by decreasing the temperature of the solvent. 12.A method of making a carbon nanotube optically visible, comprisingcoating the outer surface of the carbon nanotube with a polymer whichemits fluorescence of the visible light wavelength range.
 13. A methodof preparing a material for three-dimensional electron wiring,comprising using a polymer-coated carbon nanotube.
 14. A method ofpreparing a carbon nanotube with a high solvent dispersion property,comprising coating the outer surface of the carbon nanotube with apolymer.